Fuel cells which generate electric current by the electrochemical combination of hydrogen and oxygen are well known. In one form of such a fuel cell, an anodic layer and a cathodic layer are separated by an electrolyte formed of a ceramic solid oxide. Such a fuel cell is known in the art as a “solid oxide fuel cell” (SOFC). Hydrogen, either pure or reformed from hydrocarbons, is flowed along the outer surface of the anode and diffuses into the anode. Oxygen, typically from air, is flowed along the outer surface of the cathode and diffuses into the cathode. Each O2 molecule is split and reduced to two O−2 anions catalytically by the cathode. The oxygen anions transport through the electrolyte and combine at the anode/electrolyte interface with four hydrogen ions to form two molecules of water. The anode and the cathode are connected externally through a load to complete the circuit whereby four electrons are transferred from the anode to the cathode. When hydrogen is derived by “reforming” hydrocarbons such as gasoline in the presence of limited oxygen, the “reformate” gas includes CO which is converted to CO2 at the anode via an oxidation process similar to that performed on the hydrogen. Reformed gasoline is a commonly used fuel in automotive fuel cell applications.
A single cell is capable of generating a relatively small voltage and wattage, typically between about 0.5 volt and about 1.0 volt, depending upon load, and less than about 2 watts per cm2 of cell surface. Therefore, in practice it is known to stack together, in electrical series, a plurality of cells. Because each anode and cathode must have a free space for passage of gas over its surface, the cells are separated by perimeter spacers which are selectively vented to permit flow of gas to the anodes and cathodes as desired but which form seals on their axial surfaces to prevent gas leakage from the sides of the stack. The perimeter spacers may include dielectric layers to insulate the interconnects from each other. Adjacent cells are connected electrically by “interconnect” elements in the stack, the outer surfaces of the anodes and cathodes being electrically connected to their respective interconnects by electrical contacts disposed within the gas-flow space, typically by a metallic foam which is readily gas-permeable or by conductive filaments. The outermost, or end, interconnects of the stack define electric terminals, or “current collectors,” which may be connected across a load.
A complete SOFC system includes a combustor which burns the anode reformate tail gas in the presence of spent cathode air to reduce system emissions and to reclaim chemical energy, in the form of heat, which would otherwise be wasted. The hot combustor exhaust is then used to pre-heat air entering the fuel reformer and air being provided to the cathodes in the fuel cell stack, improving significantly the overall thermal efficiency of the system. Because the combustibles content of the tail gas can vary widely, depending upon the operating state of the fuel cells, the combustion temperature can also vary. If the fuel/air mixture is relatively lean in fuel, the resulting combustion temperature can be too low for supporting an endothermic reforming reaction, or can cause reduced efficiency in the cathode pre-heat heat exchanger. If the mixture in the combustor is relatively rich in fuel, as may happen during start-up, the combustion temperature can be high enough to generate undesirable oxides of nitrogen and/or damage the combustor components.
What is needed is a simple means for regulating combustion temperature in the tail gas combustor within a predetermined temperature range.
It is a principal object of the present invention to minimize exhaust pollutants emitted by a fuel cell system.
It is a further object of the invention to prevent internal damage by overheating of components of a solid-oxide fuel cell system.
It is a still further object of the invention to increase the efficiency of such a fuel cell system.